1. Field of the Invention
This invention relates to an apparatus for detecting a saturation gas amount absorbed by a catalytic converter such as an automotive catalytic converter disposed in an engine exhaust system. This invention also relates to a method of detecting a saturation gas amount absorbed by a catalytic converter. This invention further relates to a method and an apparatus for detecting a condition of deterioration of a catalytic converter.
2. Description of the Prior Art
It is well-known to convert harmful components of the exhaust gas of an internal combustion engine, such as HC, NO.sub.x, and CO, into nontoxic gases by means of a catalytic converter located in the engine exhaust system. Generally, the characteristics of such an automotive catalytic converter tend to decrease as it ages.
Japanese published unexamined patent application 61-286550 corresponding to U.S. Pat. No. 4,739,614 suggests an apparatus for detecting an unacceptable deterioration of an automotive catalytic converter. In Japanese application 61-28650, attention is paid to the fact that a variation of the oxygen concentration of the exhaust gas at the downstream side of a catalytic converter more clearly reflects a variation of the oxygen concentration of the exhaust gas at the upstream side thereof as the characteristics of the catalytic converter decrease. The apparatus of Japanese application 61-286550 includes an O.sub.2 sensor (oxygen sensor or probe) disposed at a position downstream of the catalytic converter. Detection is given of certain conditions where the voltage output of the O.sub.2 sensor varies above a reference voltage and the period of this variation is shorter than a reference period. In the apparatus of Japanese application 61-286550, when the number of times of the occurrence of such conditions reaches a reference number, the catalytic converter is determined to be unacceptably deteriorated.
Japanese published unexamined patent application 3-253714 corresponding to U.S. Pat. No. 5,154,055 discloses an apparatus for detecting a purification factor of an automotive catalytic converter. The apparatus of Japanese application 3-253714 includes first and second air-to-fuel ratio sensors (O.sub.2 sensors) disposed at positions upstream and downstream of the catalytic converter respectively. Detection is given of delay times of the responses of the O.sub.2 sensors to a rich-to-lean change of an air-fuel mixture. In the apparatus of Japanese application 3-253714, the purification factor of the catalytic converter is estimated from the difference between the detected response delay times. The differences between the detected response delay times are detected under two different engine operating conditions to calculate inherent response delay times of the O.sub.2 sensors. The inherent response delay times are canceled in the estimation of the purification factor to increase the accuracy of the estimated purification factor.
U.K. patent application GB 2,242,544A discloses that automotive three-way catalytic converters have a gas storage capacity (in particular, an oxygen storage capacity). The degree of conversion by the catalytic converter depends on the oxygen component in the exhaust gas which is influenced by stored oxygen delivered up by the catalytic converter. Thus, in U.K. patent application GB 2,242,544A, the degree of conversion by the catalytic converter is optimized by targeted enrichment or weakening of the ratio of air to fuel.